This invention relates to the separation of a desired isotope from polyatomic molecules containing different isotopes, by applying to the molecules in the gas phase at a predetermined pressure, near infrared radiation of a first pulsed laser and, after a predetermined time-lag which allows a sufficient number of collisions, infrared radiation of a second pulsed laser of different frequency to produce a chemical reaction resulting in a molecule, enriched in the desired isotope, which can be separated from the remainder of the material. The invention is exemplified in a particular by the separation of 13C isotopes in polaytomic molecules consisting of mostly 12C isotopes and which contain Cxe2x80x94H and Cxe2x80x94F bonds.
The stable 13C isotope has been widely used in many applications but until recently in relatively small volume. Recent medical development of the so-called carbon-13 Diagnostic Breath Test (13C DBT) (U.S. Pat. No. 4,830,010) has dramatically changed the situation. The DBTs are used to assess the condition of organs of the human digestive system. Because of its safety, relative simplicity and wide range of application, the DBT technology has rapidly increased the demand for 13C.
A limiting factor for the growth in the use of DBTs is the relatively high production cost of highly ( greater than 99%) isotopically pure 13C. The bulk of the 13C at present is produced by multi-cycle low temperature distillation of CO. This technique is well developed and has nearly reached the maximum of its efficiency, limited by its high energy consumption.
The molecular laser isotope separation (MLIS) approach provides an alternative for production of high purity stable isotopes. The most developed method for MLIS of 13C is based on infrared multiphoton dissociation (IRMPD) of CF2HCl by a pulsed CO2 laser. This method relies on a 20 cmxe2x88x921 isotopic shift in the IR absorption spectrum of the 13C containing molecules relative to 12C containing molecules for selective absorption and dissociation of 13CF2HCl. The CF2 dissociation fragments recombine, resulting in stable C2F4 molecules that are separated from parent molecules by distillation.
An example of a recent implementation of this approach by Ivanenko et al. (Applied Physics B, 62, pp. 329-332, 1996) produce a macroscopic enrichment of 13C using a high-power high repetition rate industrial CO2 laser. A report by V. Y. Baranov et al., (Proceedings of 4th All-Russian International Scientific Conference on xe2x80x9cPhysical Chemical Processes at Selection of Atoms and Moleculesxe2x80x9d, 1999, pp. 12-16) describes a near completed pilot plant in Kaliningrad, Russia, which is designed to produce several tens of kilograms of isotopically pure 13C a year using the same approach. In both cases, CF2HCl is enriched to 30-50% in 13C by selective IRMPD. In both cases, it is suggested that further enrichment of the products up to 99% 13C could be accomplished by non-laser techniques such as centrifugation. In another approach, a second stage of laser separation is employed to bring partially enriched product to higher levels of enrichment (Ph. Ma et al., Appl. Phys. B 49 503 (1989)). In the case of 13C isotope separation using CF2HCl as a starting material, the partially enriched product (C2F4) is chemically converted to a molecule suitable for the next laser isotope separation cycle (A. P. Dyad""kin, et al.; Proceedings of 4th All-Russian International Scientific Conference on xe2x80x9cPhysical Chemical Processes at Selection of Atoms and Moleculesxe2x80x9d, 1999, pp. 17-20). This extra stage complicates the overall process and significantly increases cost of the product.
Under certain conditions, single-laser IRMPD of CF2HCl has demonstrated the capability of producing products highly enriched in 13C in a single stage, but this high degree of enrichment comes at the cost of productivity. The work of Gauthier et al. (Appl. Phys. B. 28, 2, 1982) achieves enrichment to 96%, but this requires operating at low laser fluence and low pressure, both of which decrease he productivity. Reasonable productivity is achieved at only 50% 13C enrichment, which falls short of the high purity ( greater than 99%) required for medical applications.
One approach to increase the selectivity in laser isotope separation is to use a single-stage two-laser process. U.S. Pat. No. 4,461,686 relates to two-color IRxe2x80x94IR MLIS wherein a first laser excites a non-specified vibrational state and a second laser excites molecules up to a level of a chemical conversion, including dissociation. A similar method has been successfully realized on a laboratory scale by Evseev et al. (Appl. Phys. B36, 93, 1985; Sov. J. Quantum Electron. 18, 385, 1988). While this approach overcomes some of the drawbacks of a single-laser process and achieves relatively high selectivity (S=6000 which corresponds to 13C enriched to 98.5%), low pressure is still required, limiting the productivity.
One widely known problem of two-laser isotope separation schemes is the possibility of vibrational relaxation of the molecules in the time between the two laser pulses, leading to loss of isotopic selectivity, U.S. Pat. No. 4,461,686 clearly states this problem by specifying a time delay between laser pulses that is shorter than the vibrational relaxation time but longer than the rotational relaxation time of the polyatomic molecules, allowing time for rotational but not for vibrational relaxation.
A number of other two-laser schemes have been employed for separation of various isotopic species, but in most cases, conditions are adjusted to minimize collisions in the time between the two laser pulses and/or the deleterious effects of collisions on the selectivity is explicitly mentioned. In their two-color infrared isotopically selective decomposition of UF6, Rabinowitz et al. (Optics Letters 7, 212 (1982)) indicate that they use pressures of less than 10xe2x88x927 Torr during runs, ensuring collision free reactions. They clearly state that energy-exchange collisions between the two isotopic species may scramble the selectivity. Using a similar two-color laser isotope separation scheme for SeF6, Tiee and Wittig (J. Chem. Phys. 69, 4756 (1978)), state that they use a delay between the two lasers that is short enough so that deleterious energy transfer processes do not have a chance to interfere. In their two-color multiple photon dissociation of CF3T, Pateopol and O-Neil (Laser Isotope Separation, SPIE, Vol. 1859, p. 210-218 (1993)) show in FIG. 4 that an increase in pressure, which increases the frequency of collisions, decreases the isotopic selectivity. In a two laser scheme for separation of sulfur isotopes, French patent FR2530966A does not explicitly mention collisions but uses sufficiently low pressure and short time delay such that vibrational relaxation from collisions between the two laser pulses is minimized. In their two laser dissociation scheme for OsO4, Ambartzumian et al. (Optics Letters 1, 22 (1977)) do not mention collisions, however the information they provide on the experimental conditions, particularly the low pressure (xcx9c0.3 Torr) suggests that no collisional vibrational relaxation occurs during the process.
A few studies have observed that under certain conditions, collisions seem to enhance the isotopic selectivity. In their single-laser IRMPD of CF2HCl for 13C enrichment, Gauthier et al. (Appl. Phys. B. 28, 2, 1982, FIG. 3) demonstrate increasing selectivity with increasing pressure. This increase in selectivity is accompanied with a corresponding decrease in dissociation efficiency (also FIG. 3), leading to low values of productivity. In their two-laser IRMPD studies of CF2HCl for 13C enrichment, Evseev et al. (Appl. Phys. B36, 93, 1985; Sov. J. Quantum Electron. 18, 385, 1988) observe modest increase in selectivity both upon increase in the pressure of the working gas as well as upon increasing the delay between the two lasers. They attribute the increased selectivity to different rates of vibrationalxe2x80x94vibrational exchange of xe2x80x9chotxe2x80x9d ensembles of 12C and 13C containing molecules with the ensemble of xe2x80x9ccoldxe2x80x9d unexcited molecules of the main isotope, although they propose no explanation for the rate difference.
We believe that the attribution by Evseev et al. of the pressure and time-delay dependence of the isotopic selectivity to a difference in collisional deactivation rates is essentially correct, although the particular pre-excitation technique that they use, namely CO2 laser infrared multiphoton excitation (IRMPE), prohibits them from exploiting this effect for simultaneously achieving both high selectivity and high productivity in 13C isotope separation. IRMPE can either pre-excite molecules to a few low energy vibrational levels when the laser fluence is low, or to a wider distribution of higher energy levels if the laser fluence is high. In both cases, the collisional effect can provide only a limited improvement of selectivity. Indeed, the 6000 maximum isotopic selectivity in their work has been achieved only for relatively low pressure (2.5 Torr) and only for cold molecules (xe2x88x9265xc2x0 C.). Cooling molecule to temperatures in the range of xe2x88x9260 to xe2x88x9270xc2x0 C. itself typically increases selectivity of this process by a few times.
The process that is the subject of this present invention makes use of our fundamental understanding of the mechanism of isotopically selective collisional vibrational relaxation to devise a two-laser isotope separation scheme that can make optimal use of this collisional phenomenon. Our experiments show that a selectivity of greater than 9000 can be achieved at room temperature and at pressures greater than 50 Torr.
An object of the invention is to provide a two-laser infrared multiphoton dissociation process for isotope separation that can produce highly isotopically enriched species in a single stage.
According to the invention, this object is achieved by the method as set out below.
In this method, the radiation of the first laser has a predetermined frequency to excite by a single-photon a low overtone vibrational transition of the polyatomic parent molecules, in particular a hydrogen stretch vibration, to produce vibrational pre-excited molecules at a well defined energy enriched in the desired isotope, for instance 13C.
The radiation of the second laser has a predetermined frequency and predetermined energy fluence to induce selective dissociation of the vibrationally pre-excited excited molecules by infrared multiphoton excitation, in particular of a C-F stretch vibration.
The product of the pressure of the molecules and the time-lag xcex94t between the pre-excitation by the first laser pulse and the dissociation during the second laser pulse (which results from the effective length of the second laser pulse plus any time delay of the second laser pulse relative to the first laser pulse), is sufficiently high to allow collisional vibrational deactivation of a substantial amount of the vibrationally pre-excited molecules containing non-desired isotope(s), like 12C, before dissociation of the vibrationally excited molecules occurs while having no significant collisional vibrational deactivation of the pre-excited molecules containing the desired isotope, like 13C. The dissociation products are hence more highly enriched in the desired isotope as a result of collisions.
Collisions that occur between the two laser pulses and/or during the second pulse are hence used to increase significantly the isotopic selectivity.
As is described more fully below, the use of collisions to significantly increase the isotopic selectivity requires excitation by the first laser to a well defined energy of at least several thousand cmxe2x88x921. This is accomplished by direct, single photon excitation of a low overtone (xcex94v=2 or 3) of a hydrogen atom stretch vibration. The combination of low-overtone excitation by the first laser with isotopically selective collisional deactivation in the time between two laser pulses, followed by selective IRMPD of the pre-excited molecules induced by the second laser represents a unique feature of this invention.
This approach has several important advantages over other implementations of other IRMPD isotope separation schemes. First, vibrational overtone excitation with a continuously tunable laser can reach the maximum selectivity determined by the overlap in the spectra of two isotopic species, while conventional line-tunable CO2 lasers cannot be sure to hit the point of minimum spectral overlap. Moreover, isotope shifts are in general greater for overtone transitions than for vibrational fundamentals. Secondly, overtone pre-excitation of a light atom stretch vibration can promote molecules directly to the vibrational quasicontinuum with a well defined energy, allowing the parameters of the dissociating laser to be optimized for this energy, preserving the isotopic selectivity gained in the first step. Because the IRMPD processes is applied to molecules already in the vibrational quasicontinuum, efficient dissociation occurs at relatively low (0.5-3 J/cm2) CO2 laser fluence, avoiding the need to focus the CO2 laser beam. This permits a great increase of the irradiated volume, since collimated beams can be propagated together for meters, limited only by the beam divergence. While this is an important feature, it is not unique to our process. Most importantly, using vibrational overtone excitation for the first step, followed by collisions of the pre-excited molecules, enhances the isotopic selectivity of the process and at the same time allows higher working pressures where the density of molecules is higher, leading simultaneously to high selectivity ( greater than 99% isotopic purity) and reasonable productivity in a single stage process. This can make the process economically feasible and competitive with the current technologies.
Taken together, these factors indicate that the overtone excitation-IRMPD scheme according to the invention should provide a more efficient and selective means of laser isotope separation than previously developed MLIS schemes. The results described below demonstrate that this is indeed the case.
As mentioned above, isotope separation can operate with low fluence laser beams enabling interaction by multiphoton dissociation over a large volume. Consequently, the first and second laser beams can be collimated or slightly diverging or slightly converging beams of low fluence (xe2x89xa65 J/cm2) overlapping with one another over a substantial portion or all of their respective volumes containing the said polyatomic molecules. The first and second beams can have an angle of divergence/convergence less than 2.0xc3x9710xe2x88x923 rad.
The method according to the invention is particularly advantageous for separating 13C isotopes from polyatomic molecules consisting of mostly 12C isotopes and which contain Cxe2x80x94H and Cxe2x80x94F bonds, for example molecules of the formula HCF2X, wherein X is F, Cl, B or I. There have been a number of other papers and patents that share these working molecules but use a completely different process which does not use collisions to enhance selectivity. We do not claim this class of compounds for laser isotope separation in general, but only as suitable candidates under the specific conditions of our process.
In one example, the molecules are trifluoromethane HCF3, the frequency of the first laser is 8753xc2x11 cmxe2x88x921 or 8549xc2x11 cmxe2x88x921, the frequency of the second laser is in the range 1020-1070 cmxe2x88x921, and the predetermined energy fluence of the second laser has a value in the range 0.5-5 J/cm2 depending on the pulse shape of the second laser. Alternatively, for trifluoromethane HCF3, the frequency of the first laser is 5936.5xc2x11 cmxe2x88x921 or 5681xc2x11 cmxe2x88x921.
Alternatively, the molecules are CF2HCl and the predetermined frequency of the first laser is 5911xc2x15 cmxe2x88x921 or 8693xc2x12 cmxe2x88x921, or the molecules are monofluoromethane CH3F.
The first predetermined frequency can be produced by stimulated Raman scattering of narrowband tunable radiation of a solid state pulsed laser and the second predetermined frequency is produced by a pulsed CO2 laser.
The method of the invention can also be applied to the separation of isotopes from other molecules including SiH4, SiF3H, SiCl3H, GeH4, and alcohols of the formula Rxe2x80x94OH, where R=CH3, C2H5, C3H7 or C4H9.
The overlapping first and second laser beams can be substantially parallel or can multiple intersect.